The book I’m looking for is a YA book about a teenage boy who travels to the “future” but it’s primitive world after some sort of disaster? I think a bomb went off. My memory is fuzzy here. He lives out a whole life, joins the local tribe, even learns the language. Then, suddenly he is transported “back” to his own time to a mall. He ends up growing up to be a scientist in the hope of stopping whatever nuclear disaster or climate disaster that happens.
Lost Classics of Teen Lit: Name That Book! 